1. Field of the Invention
This invention relates to a real image mode variable magnification finder having an objective optical system, an ocular optical system and a plurality of reflecting surfaces for erecting an image of an object to be observed.
2. Description of the Related Art
In contrast to a single lens reflex camera, a compact size camera such as so-called lens-shutter cameras has a finder separate from its picture taking optical system. As a finder optical system, a real image mode finder which has an objective optical system, an ocular optical system and a plurality of reflecting surfaces for erecting an image of an object to be observed is well known.
In recent years, the need has become more and more intense in the marketplace for a camera having a high zooming ratio and small body, particularly in the light entering direction.
Generally speaking, in order to increase the zooming ratio, a zoom lens must have lens units having strong optical powers or large amounts of movement in the direction of an optical axis. However, the ability to strengthen the optical power is limited because it causes large aberrations. Therefore, to increase the zooming ratio, it is inevitably to increase the amount: of movement of the movable lens units. This results in a thick camera body. For a picture taking optical system, a so-called collapsible lens mechanism can be used in which the lens barrel is telescopically collapsible into the camera body when the camera is not in use. This helps to miniaturize the camera thickness.
However, using a collapsible mechanism for the finder optical system is not desirable for camera styling and the thickness thereof nowadays has become one of the major hurdles to miniaturize the camera body.
On the other hand, to attain high zooming ratio and small change in aberration with zooming operation, the number of lens units in the finder optical system must be increased.
This causes a large sum total of lens thickness and therefore, is undesirable with regard to miniaturizing the thickness of the camera body.
Japanese laid open unexamined patent publication Hei 5-53,054 (counterpart U.S. Pat. No. 5,231,534 and No. 5,323,264) discloses a conventional finder optical system having an objective lens system including a first negative lens unit, a second positive lens unit and a third negative lens unit in this order from an object side, in which the first and second lens units are movable for zooming. The third lens unit is fixed during zooming operation and includes a lens or a prism having a negative optical power on the entrance surface thereof. The objective lens system includes a negative lens unit at rearmost side thereof (pupil side of the user) and has small thickness, obtained by adopting a so-called telephoto type power arrangement. U.S. Pat. No. 5,231,534 discloses, particularly in FIG. 2 and FIG. 7, a finder optical system having an objective lens system made up a first negative lens unit, a second positive lens unit, a third positive lens unit and a fourth negative lens unit in this order from an object side, in which the second and third lens units are movable for zooming. This finder optical system also uses the so-called telephoto type power arrangement.
Japanese laid open unexamined patent publications Hei 5-93,863, Hei 6-300,971, and Hei 7-13,076(counterpart U.S. Pat. No. 5,627,618) discloses a finder optical system having an objective lens system including a first positive lens unit, a second negative lens unit, a third negative lens unit and a fourth positive lens unit in this order from an object side, and in which the second and third lens units are movable for zooming. A partial optical system including the first through third lens unit and having a negative optical power as a whole forms a virtual image of an object and the fourth fixed lens unit forms a real image of the object.
On the other hand, a real image mode finder optical system is proposed in which a reflecting surface for erecting an image has an optical power and also participates in image formation of an objective lens system and/or image magnification of an ocular lens system for miniaturizing the size thereof. The reflecting surface is embodied in two types of optical elements—one is a reflecting mirror and the other is a reflecting prism. These two types of optical elements have different optical properties. The optical power of a reflecting surface of a prism having radius of curvature r is −2n/r where n is a refractive index of the incident and exit side of the reflecting surface. On the other hand, the optical power of a reflecting surface of a mirror is −2/r which is different from the optical power of the prism. The same optical power can be obtained by smaller curvature in a prism than in a mirror. Therefore, a prism is advantageous to reduce aberrations. In addition, a prism has two refracting surfaces, an entrance surface and an exit surface, in addition to the reflecting surface. These three surfaces all contribute to correction of aberrations and therefore, a prism is further advantageous for reducing aberrations as compared to a mirror. Further, a prism is made of a medium having refractive index more than 1 and a larger optical path length can be obtained with a prism than with a mirror.
On the other hand, a reflecting surface is more sensitive to decentering error and requires higher assembly precision than a refracting surface. In the finder optical system having a plurality of reflecting surfaces, a prism is advantageous because the plurality of reflecting surfaces can be arranged on a single optical element and the positional relationship therebetween is more easily set precisely in a prism optical system than in a mirror optical system.
As is apparent from the foregoing, a prism is superior to a mirror when a reflecting surface has a curvature and an optical power.
Japanese laid open unexamined patent publication Hei 8-248,481 (counterpart European patent application No.722,106) discloses a real image mode finder optical system including a prism disposed on the side of an objective optical system having a rotationally symmetric curved aspherical reflecting surface. The objective lens systems thereof have variable focal lengths in the illustrated embodiments.
Japanese laid open unexamined patent publication Hei 10-197,796 discloses a large number of embodiments of a real image mode finder optical system using a rotationally asymmetric curved surface in the prism having an image erecting reflection surface in order to obtain a small and high performance finder. Among them, however, only six embodiments are actually designed in detail by conducting a light ray tracing. They are all directed to miniaturizing a finder by incorporating the function of an ocular optical system to the prism. Only the sixth embodiment includes numerical data for an objective optical system but no rotationally asymmetric surface is used in an optical system forming a real image of an object.
Japanese laid open unexamined patent publication Hei 10-333,041 discloses a real image mode finder optical system comprising a prism disposed on the side of an objective optical system having a rotationally asymmetric curved reflecting surface. The objective lens system thereof has a fixed focal length. Japanese laid open unexamined patent publications Hei 11-38,472 and Hei 11-38,473 disclose a real image mode variable magnification finder including a variable magnification objective lens system having a zooming ratio of 2.5 to 3.5. A rotationally asymmetric reflecting surface is provided on a prism disposed on an object side in a Porro prism for erecting an image.
Japanese laid open unexamined patent publication Hei 11-84,247 discloses a real image mode finder including a prism having a rotationally asymmetric reflecting surface. In this finder optical system, an image of an object is erected by relaying an image formed by an objective lens one more time with a relay lens system. An optical axis which enters the objective lens system and that which exits from the ocular lens system are approximately parallel with each other. The objective lens system has a fixed focal length.
The above described conventional finder optical systems involve drawbacks mentioned below.
In the finder optical system disclosed in Japanese laid open unexamined patent publication Hei 5-53,054, as the optical power of the third negative lens unit becomes strong, the positive power of the second lens unit also necessarily become strong. In the finder optical system disclosed in U.S. Pat. No. 5,231,534, as the optical power of the fourth negative lens unit becomes strong, the composite positive optical power of the second and third lens units also necessarily becomes strong. These conditions make it difficult to attain good imaging performance. Therefore, there is a limit to the ability to reduce the length (thickness) of the finder by means of increasing the optical powers of each lens units. Moreover, as a large back focal distance is required to place an optical element having an optical power after the third lens unit, a so-called inverted telephoto type power arrangement is adopted for the objective lens system. This makes it difficult to reduce the length of the finder optical system.
In Japanese laid open unexamined patent publications Hei 5-93,863, Hei 6-300,971 and Hei 7-13,076, the structure of the fourth lens unit is very important because the virtual image formed by the first through third lens units is magnified by the fourth lens unit. However, the fourth lens unit must be positioned near the object side in order to maintain a long back focal distance. Therefore, a magnification power shared by the fourth lens unit is limited to some extent and miniaturization of the finder is unsatisfactorily achieved. Moreover, the fourth lens unit is including a single lens element and has a limited ability for correcting aberrations. This is disadvantageous when it is desired to make the zooming ratio high.
It is possible to achieve both high zooming ratio and good imaging performance by constructing the fourth lens unit of a plurality of lens elements. However, this will result in a thick fourth lens unit and is disadvantageous in regard to miniaturizing the finder.
In the zoom optical system, the lens configuration and power layout for each lens unit, especially the movable lens units are important to obtain a zoom optical system that is small and has a high zooming ratio. As a zoom optical system and a fixed focal length optical system are very much different in structure and character, even when technologies useful for an fixed focal length optical system are applied to a zoom optical system, the same effects and advantages are not always obtainable.
Japanese laid open unexamined patent publications Hei 10-333,041 and 11-84,247 do not disclose a zoom optical system nor suggest applying the optical system disclosed therein to a zoom optical system. On the other hand, Japanese laid open unexamined patent publications Hei 8-248,481 discloses a zoom optical system but the imaging performance is limited to a conventional level. Therefore, a zoom optical system having high zooming ratio which is small in size will be difficult to be achieved even when considering these prior art.
The finder optical systems disclosed in Japanese laid open unexamined patent publications Hei 11-38,472 and 11-38,473 are unsatisfactorily miniaturized because the type of the objective lens system, the number of reflecting surfaces, the optical power arrangement, or the like, are not optimized. Besides, the zooming ratios obtained by ray tracing of the disclosed embodiments are within a range from around 2.1 to 2.8 and not satisfactorily large.
As mentioned above, these conventional finder optical systems do not satisfy the requirement of miniaturization or high zooming ratio and there is no finder optical system to realize these two requirement at the same time.